1. Field of the Invention
The present invention relates to a static semiconductor memory device, for example, a static random access memory (SRAM).
2. Description of the Related Art
A general SRAM composed of six MOSFETs is made up of pull-down transistors (driving transistors) each of which is connected to a storage node and the ground at both ends thereof, load transistors each of which is connected to a storage node and the power source at both ends thereof, pass gate transistors (transfer transistors) each of which is connected to a storage node and a bit line at both ends thereof. In order to enhance stability at the time of data reading of an SRAM, it is necessary to make the driving force of the pull-down transistor larger than that of the pass gate transistor, thereby preventing the voltage of the storage node from exceeding the threshold of the pass gate transistor.
Heretofore, in order to make a difference in driving force between the pull-down transistor and the pass gate transistor, a method has been used in which the Lw/Lg ratio of the pull-down transistor is made larger than that of the pass gate transistor. However, reduction in the cell-size of the SRAM is required with the times. Hence, it has become difficult to make a difference in gate width between these transistors.
Only for the purpose of enhancing the stability at the time of data reading of the SRAM, it is possible to make the driving force of the pass gate transistor degraded. However, when data is written, driving force is required of the pass gate transistor, and hence it is inexpedient to employ this method.
Thus, a method has been developed in which an increase in the cell area is suppressed, resistance is connected to the pass gate transistor and the storage node at both ends thereof, the read data is prevented from being corrupted even when the word line is made thin (see for example, Jpn. Pat. Appln. KOKAI Publication No. 05-304274).
However, it is required to further suppress an increase in the cell area, improve write characteristic, and secure stability at the time of data reading.
Thus, a static semiconductor memory device in which write characteristic is improved by preventing the cell area from increasing, and stability at the time of data reading can be secured is demanded.